HARRISON, III, WILLIAM B. and ATEKWANA, ESTELLA, Dept. of Geology, Western Michigan University, Kalamazoo, MI

Michigan has produced nearly 1.2 billion barrels of oil and over 4.5 trillion cubic feet of natural gas since the early 1920's. Virtually all that production is from strata of Middle Ordovician through Mississippian age. Drilling of deeper wells since 1980 has provided additional data about reservoir rocks, source beds and structures. These new wells have provided modern wireline logs, detailed stratigraphic data, and occasional core or reservoir engineering tests. It is now possible to make some generalizations about the hydrocarbon potential of the deep, old Lower Ordovician and Cambrian sedimentary section in Michigan.
Examination of modern logs, drill cuttings and limited core samples confirm that the Lower Ordovician and Cambrian strata in Michigan are lithologically similar to the classic Sauk Sequence strata described from the outcrop regions in the upper Mississippi valley. Although the lithofacies are similar there are significant differences in thicknesses and petrographic properties of Michigan's rocks due to their deposition in a rapidly subsiding cratonic basin. Significant source rocks have not been identified within the Sauk Sequence strata in Michigan.
Diagenesis due to burial is the primary factor that affects reservoir properties. Intergranular porosity in the sandstones is open in the shallow parts of the basin and occluded with quartz and carbonate cements on the deeper basin. Fracture and solution-enlarged porosity form the reservoir in the carbonate intervals tend to be concentrated closer to the basin margins.
Potential traps are likely to be anticlinal structures associated with basement faults which upon reactivation propagate upward through the sedimentary section. Seismic reflection profiles and gravity and magnetic data may help identify potential traps and basement structures.

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